1. Field of the Invention
The present invention relates to a photosensitive resin composition for forming a polyimide film pattern used as an insulating material of an electronic part or as a passivation film, an .alpha.-ray shielding film or an interlayer insulating film in a semiconductor device. The present invention also relates to a method of forming the polyimide film pattern.
2. Description of the Prior Art
In a semiconductor device, a protective film (passivation film) is formed on the surface of a semiconductor substrate having a semiconductor element formed therein in order to protect the element from the influence of the external environment and to improve the reliability of the element. In general, a polyimide resin excellent in its electrical properties such as the insulating properties and in its resistances to radiation and heat is widely used as a material of the protective film. These excellent properties also allow the polyimide resin to be used widely as a material of the .alpha.-ray shielding film in a semiconductor device and as a material of the interlayer insulating film included in a multilayer interconnection structure.
A polyimide resin film can be easily formed from its precursor of a polyamic acid. Specifically, a predetermined surface is coated first with a varnish of the polyamic acid, followed by heating the coated layer so as to form a film. Then, a heat treatment is applied to the film so as to subject the polyamic acid to a cyclizing reaction. As a result, the polyamic acid is cured (imidized) so as to form a polyimide film. In this method, a stabilizing treatment can be performed at a relatively low temperature and, thus, the method is employed widely.
On the other hand, a pad processing or the like is required in the manufacture of a semiconductor device in order to make a through-hole in a multilayered interconnection structure or to achieve an electric connection to an external lead wire. Thus, it is necessary to pattern a polyimide film formed as a protective film or an interlayer insulating film in a semiconductor device so as to form a hole having a predetermined pattern structure. In general, PEP (photo engraving process) using a photo resist is employed for patterning a polyimide film. Specifically, a polyimide film is formed first as described previously on the surface of a semiconductor substrate having a semiconductor element formed therein, followed by forming a photo resist film on the polyimide film. Then, the photo resist film is selectively exposed to light and subsequently developed so as to form a resist pattern. Further, the polyimide film below the resist pattern is selectively etched using the resist pattern as an etching-resistant mask so as to form a polyimide protective film or an interlayer insulating film of a desired pattern.
In the conventional method of forming a polyimide film pattern described above, however, it is necessary to perform formation of a polyimide film and PEP for the pattern formation as two independent processes. Naturally, the two-stage process makes the formation of the polyimide film pattern troublesome.
To overcome the difficulty noted above, proposed is a resin composition containing a precursor of a polyimide, said composition making it possible to pattern a polyimide film without relying on PEP. For example, Published Unexamined Japanese Patent Application No. 49-115541 discloses a polyamic acid ester prepared by the reaction between a dichlorinated dicarboxyl diester having a photopolymerizable group introduced therein by an ester bond, which is used in place of the tetracarboxylic dianhydride which is used generally, and a diamine. The polyamic acid ester exhibits a negative photosensitivity. Specifically, when exposed to light, the exposed portion of the composition itself is made insoluble in a developing solution. It follows that patterning can be achieved simultaneously with formation of a polyimide film. In other words, a polyimide film pattern can be formed without relying on PEP.
However, very troublesome operations are required for synthesizing the compound having a photopolymerizable group and for synthesizing the polyamic acid ester. An additional difficulty is that chlorine ions are contained as an impurity in the final product resin. Further, an organic solvent is used as a developing solution, with the result that it is necessary to use a large amount of the organic solvent in the manufacturing process of the semiconductor device. This is not desirable in terms of safety, sanitation and environmental contamination. What should also be noted is that the formed polyimide film pattern is swollen by the solvent, leading to a low resolution.
Published Examined Japanese Patent Application No. 59-52822 discloses a heat resistant photosensitive material (or a negative photosensitive resin composition) containing a compound having a carbon-to-carbon double bond capable of dimerization or polymerization by actinic radiation and an amino group. However, when the photosensitive material disclosed in this prior art is used for forming a protective film in a resin-encapsulated type semiconductor device or the like, the protective film is low in its adhesion with a semiconductor substrate, a protective film of an inorganic material on the substrate surface or with the encapsulating resin, with the result that the reliability of the semiconductor device is impaired. Further, it is necessary to use an organic solvent as a developing solution in the developing step, as in the case of using the polyamic acid ester referred to previously. It follows that various problems are brought about, as pointed out previously.
Published Unexamined Japanese Patent Application No. 60-6365 discloses a photosensitive resin composition containing a compound prepared by adding as a salt an aminomethacrylate to the carboxyl group of a polyamic acid. However, the photosensitive resin composition disclosed in this prior art is defective in that the composition is low in solubility in a solvent used.
On the other hand, Published Unexamined Japanese Patent Application No. 62-145240 discloses a photosensitive resin composition containing a polymer having an isoimide structure. The composition exhibits a positive photosensitivity. Specifically, when exposed to light, the exposed portion of the composition is made soluble in a developing solution. However, the polymer is low in its heat resistance and in sensitivity to light, making it impractical to use the composition for forming a protective film included in a semiconductor device.
A photosensitive resin composition exhibiting a positive photosensitivity is also disclosed in Published Unexamined Japanese Patent Application No. 64-60630. Specifically, disclosed in this prior art is a composition prepared by adding a photosensitive agent of o-quinone diazide compound to an imide soluble in a solvent, said imide being synthesized by the reaction between a diamine having a hydroxyl group bonded to the aromatic ring and an acid anhydride. Further, Published Unexamined Japanese Patent Application No. 62-135824 discloses a similar photosensitive resin composition containing a quinone diazide as a photosensitive agent. Still further, Published Unexamined Japanese Patent Application No. 60-37550 discloses a photosensitive precursor of the polyimide having an o-nitrobenzyl ester group.
An alkaline aqueous solution can be used for the development of the photosensitive resin composition of a positive photosensitivity. Naturally, the problems brought about by the use of an organic solvent need not be worried about. However, the construction of the polymer, which is the main component of the resin composition, is restricted in the case of the positive photosensitive resin composition, resulting in a low adhesion with a silicon wafer, glass substrate, ceramic substrate, encapsulating resin, metal, etc. which are widely used in a semiconductor device.
In addition, Published Unexamined Japanese Patent Application No. 52-13315 discloses a positive photosensitive resin composition which can be developed with an alkaline aqueous solution, i.e., a photosensitive precursor of the polyimide which utilizes the solubility in an alkaline solution of a polyamic acid and contains a naphtoquinone diazide compound as a dissolution inhibitor. The photosensitive precursor of the polyimide permits the pattern formation with a simplified process of film formation, exposure and development. However, the difference in solubility in an alkaline developing solution is not sufficiently large between the exposed portion and the non-exposed portion in the developing step, making it difficult to form a fine polyimide film pattern. Further, Published Unexamined Japanese Patent Application No. 62-135824 referred to previously teaches that a photosensitive polyimide precursor film is heated to about 90.degree. C. before exposure to light so as to improve the resolution of the formed polyimide film pattern. Even if the particular process is applied, however, the resolution remains to be insufficient in the case of using the photosensitive precursor of the polyimide noted above.